The card that blackjack dealers (among other casino games) discard during active play. Showing the card after a hand is considered poor etiquette as the card could have changed the outcome of the hand in the player's favor. The same principle applies to a girl telling a guy, who likes her, (or vice versa) that he would've had a chance with her, had certain circumstances were in affect. Making a heartbreaking outcome even more devastating.

Jack: "Hi, Jill. Would you like to go out this weekend?"
Jill: "Sorry, I have a boyfriend."
Jack: "Oh, I didn't know. No problem."
Jill "Yea we're talking about getting married. But you're totally awesome. If I didn't have a b/f, I woulda SOOOO gone out with you."
Jack: "Sweetie, never show the burn card."